In general, the ink composition for use in writing instruments are divided into two groups: the first is an oil base or organic solvent base ink composition, and the second is a water base ink composition. The oil base ink composition has an advantage that it can write on an impervious surface such as of glass or resin as well as on a pervious surface such as of paper. However, the oil base ink composition usually contains an aromatic hydrocarbon such as xylene or toluene as a solvent, and consequently there is a fear that such a solvent might be injurious to the health. In addition, the oil base ink composition has a disadvantage that when it is used to write on paper, it permeates through the paper to the back. In turn, the water base ink composition has no such a problem as involved in the oil base ink composition, however, the water base ink composition provides writing having no water resistance, and besides it does not write on a impervious surface such as of glass or resin.
Accordingly, in the production of water base ink composition containing a dye as a colorant, the employment of a direct dye, or an additive such as a wetting agent has been proposed in order to make writing formed with a water base ink composition water resistant. There has also been proposed a water base ink composition which employs a pigment as a colorant which is originally water resistant.
However, it has been found that a water base ink composition containing a dye as a colorant provides no writing of sufficient water resistance even if it is used to write on a pervious surface such as of paper. On the other hand, with an oil base ink composition, it has been found difficult to disperse a pigment evenly in a solvent so that an oil base ink composition has a trouble in storability, and also in multicoloration.
A drawing ink for use in designing has been known which is water base and yet water resistant, and which can write on an impervious surface such as of glass or resin. The ink composition comprises an aqueous solution of shellac resin and a basic dye dissolved therein. However, the shellac resin is essentially a thermosetting resin so that when the drawing ink is left standing over a long period of time, the polymerization of shellac resin takes place to form precipitates tn the ink composition. Furthermore, when the shellac resin is hydrolyzed with an alkali to produce a resin soap in the ink composition, and then the permeability of the ink composition into paper becomes larger as time passes. Consequently, there arises a problem of change in color of writing when it is used to write on paper.
The drawing ink contains a basic dye in an amount of less than 1.0 part by weight in relation to 10 parts by weight of shellac resin. Thus, the drawing ink usually contains a dye insufficiently so that it fails to form writing of sufficient darkness when used in a felt pen type writing instrument. Moreover, when the drawing ink is left standing for a few days in the felt pen type writing instrument, it provides a faint and patchy writing, and can not be suitably used in a felt pen type writing instrument.